I suffered from depression at 19, a little older than your dd. At the time, when you are in it, it seems like it is impossible to imagine being better, life being normal let alone good, but have confidence that your dd can and will get over it.
You can help by showing her you care (which I'm sure you do anyway); ensuring she gets appropriate medical care (anti-depressants, counselling, whatever - though be aware that some counsellors are crap and not all ADs work for everyone and all take a few weeks to start being effective); ensuring her basic physical needs are being met so her body/brain can help to heal themselves, eg trying to ensure she eats highly nutritious food and vitamins, gets exercise and daylight (even if it's the last thing she feels like) and sleep if possible; and try to remove stresses from her life. Teens' lives are so much more stressful now, I think, than they were in our day - stresses about grades, money, jobs, appearance, cyber-bullying etc. I have no idea what underlying factor/s are behind your dd's depression but there will be some, and it will help her if she can work out what they are and get things in perspective. That's unlikely to happen suddenly, in an 'aha' moment - more of a gradual straightening-out of her head.
Looking back, I don't view my depression negatively or as an aberration - it was a natural result of the stresses I was under and forced me to sort things out and get things in perspective. So try to view even this stage as a 'positive' one in the sense that your dd can move on from this in a stronger and better way.
Good luck.
Oh, and very very important, don't forget to look after yourself, too - enough food, sleep, exercise, talking, relaxing etc as well - as you also need support now too, to be able to support your dd effectively.